1. Field of the Invention
The present invention relates generally to a perpendicular magnetic recording disk drive, and more particularly to a head positioning control operation that is performed based on servo data that is perpendicularly magnetically recorded.
2. Description of the Related Art
Practical use of disk drives of perpendicular magnetic recording has recently been promoted. In the perpendicular magnetic recording, generally, servo data used for head positioning control is recorded on a disk medium (referred to as a disk hereinafter) by a write head that is called a single pole type (SPT) head suitable for perpendicular magnetic recording.
As is known, the servo data is broadly divided into a cylinder code (track address) for identifying a track (cylinder) on the disk and a servo burst signal (sometimes called burst data) for sensing a position of the head within each track.
A disk drive incorporates a head positioning control system using a CPU as a main component. This system puts a read/write head in a target position (target track position) on a disk based on servo data that is reproduced by a read head (which is normally made of a GMR element) of the read/write head.
If no servo data can be reproduced correctly, it is naturally difficult to position a head correctly. As a measure to avoid an error in reading a servo mark included in servo data, a method of optimizing a sense current of a GMR element that configures a read head is proposed (refer to Jpn. Pat. Appln. KOKAI Publication No. 11-66509, for example).
Since the read/write head is configured to perform its read/write operation, there exists an offset between head elements. A method of offsetting in head positioning mode using a pre-measured offset value as well as servo data is proposed (refer to U.S. Pat. No. 5,978,168 (1999) and U.S. Pat. No. 6,067,205 (2000), for example).
In a perpendicular magnetic recording disk drive, the following phenomenon is confirmed: If a single-pole write head applies a magnetic field of recorded data whose polarity is opposite to the magnetization polarity of data magnetically recorded in advance on a disk, the recorded data changes in magnetization state. This phenomenon is called a hard easy transition shift (sometimes referred to as HETS hereinafter) or simply called a hard transition (see, for example, “200 kFRPI Data Recording Using a Novel Bi-Layered Head in Perpendicular Magnetic Recording,” IEEE Transaction on Magnetics, Vol. 29, No. 6, November 1993, Page 4074). In the HETS, when a head writes data whose magnetization polarity is opposite to that of a disk on which data has not yet been recorded, an apparent magnetic field viewed from the head becomes strong and a magnetization displacement point is shifted.
In the above perpendicular magnetic recording disk drive, the HETS phenomenon is confirmed in which a magnetization state of data changes when the data is magnetically recorded on the disk by the write head.
In the perpendicular magnetic recording disk drive, a head positioning control system compares a cylinder code reproduced from servo data read by the read head and a target cylinder code with each other in head positioning mode. In this comparison, the occurrence of errors in comparison due to the HETS phenomenon is confirmed by a performance evaluation test on the disk drive. A number of errors in comparison occur particularly in cylinder codes of tracks at the inner radius of the disk where the recording density of servo data increases.
In general, a track that causes an error in comparison between cylinder codes is registered as a defective track at the time of manufacture and not used for recording of data and, in other words, a so-called alternate track assigning operation is performed. However, a disk drive with defective tracks exceeding a prescribed number is treated as a defective one.
Consequently, the manufacturing yields of disk drives deteriorate if a number of errors in comparison between cylinder codes occur. Since, moreover, an alternate track assigning operation is required if an error in comparison between cylinder codes occurs after shipment of products, the performance of write operations decreases.